1. Field of the Invention
This invention relates to a process for purifying crude isoindolinone series pigments. More particularly, it relates to a process for producing isoindolinone series pigments which are characterized as exhibiting extremely enhanced purity, color definition and chroma, as well as good fastness to light, chemicals and weathering, and furthermore, being uniform in particle size and possessing good dispersibility in vehicles.
By the term "isoindolinone series pigments", used above, is meant pigments represented by the general formula: ##STR3## wherein
R represents a carbocyclic bridge member such as:
an unsubstituted phenylene or diphenylene group,
a phenylene or diphenylene group substituted by chlorine atoms, methyl groups or methoxy groups, or
a group expressed by the formula ##STR4## where X is oxygen or a methylene group; or
R represents a heterocyclic bridge member such as:
a pyridylene group; or
R represents a direct bond.
These isoindolinone series pigments are hereinafter referred to as "isoindolinone pigments" for brevity.
2. Description of the Prior Art
Isoindolinone pigments are useful as organic pigments of a high quality. The processes of producing isoindolinone pigments are described in, for example, U.S. Pat. Nos. 2,973,358; 3,076,815 and 2,537,352. Most isoindolinone pigments produced by these and other known processes contain trace amounts of unreacted starting compounds and side-reaction products, and therefore, are not completely satisfactory in their color definition, chroma and fastness to light, chemicals and weathering.
In general the following processes are popularly employed for the purification of organic pigments. Namely, in one process, organic pigments are dissolved in a concentrated acid and, then the solution is diluted thereby to precipitate pigments. For example, quinacridone pigments and phthalocyanine pigments are dissolved in concentrated sulfuric acid and, then, the solution is diluted with water or an alcohol. In another process, organic pigments are converted to a salt form followed by the hydrolysis of the salt by using an acid. For example, quinacridone pigments are treated with an alcoholic potassium hydroxide solution, thereby to form their potassium salts, and then, the potassium salts are hydrolyzed by an acid.
Some proposals have also heretofore been made for the purification of isoindolinone pigments. For example, U.S. Pat. No. 3,758,497 discloses a process wherein an isoindolinone pigment is dispersed in a hydrophilic organic solvent; a base capable of forming a salt is incorporated in the dispersion, thereby to form a salt of the pigment, and; then, the pigment salt is hydrolyzed by means of acid or water in the presence of such amounts of a hydrophilic organic solvent that an extensive degree of dissolution of the pigment salt occurs. The pigment purified by this proposed process is characterized as being in a very finely divided form and possessing excellent transparency. However, this pigment is poor in its hiding power and dispersibility in vehicles, and when it is used in paints, the paints are undesirably highly viscous. Furthermore, in order to obtain a high degree of purification, the proposed process requires the use of an expensive hydrophilic organic solvent, such as dimethylformamide or N-methylpyrrolidone. Such solvent exhibits too high a boiling point so that recovery of the solvent cannot be performed without using a complicated operation. In addition, since a hydrophilic organic solvent is miscible with water, complicated equipment and operations are necessary for its recovery and waste water disposal. Furthermore, when the starting crude isoindolinone pigment is obtained in the dispersed form in a hydrophobic organic solvent (e.g. as described in U.S. Pat. No. 2,973,358), the replacement of the hydrophobic organic solvent by a hydrophilic organic solvent is necessary.